Wastewater treating processes usually include multiple treatment areas or zones which can be roughly broken down into: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area.
The wastewater treatment process begins with the preliminary treatment area. Preliminary treatment is concerned with removing grit and damaging debris, such as cans, bath towels, etc., from the untreated wastewater. This is usually a two-stage treatment process whereby the debris such as rags and cans are removed by screens and the grit and heavier inorganic solids settle out of the untreated wastewater as it passes through a velocity controlled zone. The damaging inorganic debris is thus removed by screening or settling while organic matter carried within the fluid stream passes on.
Following the preliminary treatment area, the wastewater is directed to a primary treatment area. The primary treatment area entails a physical process wherein a portion of the organics are removed by flotation or sedimentation. The organics removed include feces, food particles, grease, paper, etc. and are technically defined as suspended solids. Usually 40-70% of the suspended solids are removed in this primary stage.
The third treatment stage is called secondary treatment and is usually a biological treatment process where bacteria are utilized under controlled conditions to remove nutrients or nonsettling suspended and soluble organics from the wastewater. These materials would result in an unacceptable biological oxygen demand (BOD) if left untreated. Typically, one mode of this process consists of a basin in which the wastewater is mixed with a suspension of microorganisms. This mixture is then aerated to provide oxygen for the support of the microorganisms which may then adsorb, assimilate, and metabolize the excess biological oxygen demand in the wastewater. After sufficient retention time, the mixture is then introduced into a clarifier or settler into which the biomass separates as settled sludge from the liquid. The purified fluid then overflows into a receiving stream.
There are three principal types of secondary treatment for effecting treatment of wastewater. The first type, known as a trickling filter, allows the wastewater to trickle down through a bed of stone whereby the organic material present in the wastewater is oxidized by the action of microorganisms attached to the stone. A similar concept is the RBC or rotating biological contactor wherein the biology is attached to the media which rotates in the wastewater and purifies it in the manner of a trickling filter. The second method is an activated sludge process in which the wastewater is fully aerated and agitated by either compressed air or mechanical means together with a portion of the biomass which has been returned from the clarifier or settler. The third process may be referred to as a semi-aerobic (anaerobic/oxic) process in which the first stage is anaerobic or anoxic and is followed by an oxic stage. This anaerobic-oxic-anoxic process is very similar to the initial stages of the Phoredox process and the modified Bardenpho process, both well known in the wastewater treatment industry.
This anaerobic-oxic process was first disclosed in U.S. Pat. Nos. 2,788,127 and 2,875,151 to Davidson which issued in 1957 and 1959, respectively. In the anaerobic-oxic process, the untreated wastewater is first subjected to anaerobic treatment and then to aerobic decomposition. A portion of the sludge formed during the aerobic decomposition is recycled back and mixed with the untreated wastewater being subjected to anaerobic treatment. Davidson noted that the aerobic organisms in the recycled activated sludge are not impaired by passage through the anaerobic reactor an may, in fact, undergo unusual stimulation. Heidi and Pasveer confirmed the work of Davidson in 1974 and found that soluble BOD.sub.5 removal occurred in the anaerobic zone.
In recent years, there has been a great deal of work directed at biological processes for removing pollutants such as phosphorus and nitrogen (TKN) from wastewater. This work has in large part been broadly based and has not focused on specific problems and concerns. For example, many wastewater facilities are now facing very stringent phosphorus control standards. When there is already a wastewater treatment facility in place, it becomes prudent to consider the possibility of modifying these existing facilities in order to meet new standards being imposed. Obviously costs, both initial and operating, are of main concern. One important concern then is to evaluate the economics of modifying existing treatment facilities to accomplish biological phosphorus removal.
Beyond the problem of modifying an existing wastewater facility to accomplish effective and efficient biological phosphorus removal, there are certain unique or special problems that can be introduced into the process simply because of the geographical location of the wastewater treating facility and, the particular biological process currently being practiced. These special problems have not been addressed. In this regard, there are certain situations where the wastewater that is being subjected to secondary treatment has a relatively low BOD to phosphorus ratio, that is, a ratio within the range of 7-14. This presents a special problem in biologically removing phosphorus from such wastewater. It is generally appreciated that the higher the BOD to phosphorus and BOD/TKN ratio the easier it is to biologically remove phosphorus from wastewater. Thus, in some geographical locations, where the BOD content of the wastewater is relatively low, it is more difficult to create a favorable environment for the phosphorus consuming microorganisms and consequently, it is more difficult to biologically remove phosphorus. The difficulty is so pronounced that some commercially available processes that claim to biologically remove phosphorus from wastewater will not even warrant their process in wastewater conditions where there is such a relatively low BOD to phosphorus and TKN ratio. The Bardenpho process requires a BOD.sub.5 /TKN ratio of 6:25 or higher and the UCT process requires more than 3.6:1.0 ratio, preferrably 5 or higher to assure phosphorus removal.
Therefore, there is a need for a biological phosphorus removal process that is particularly designed and suitable for incorporation into an existing conventional wastewater facility. Further, there is a need for an efficient and effective biological phosphorus removal process that is capable of working with wastewater that has a relatively low BOD to phosphorus and TKN ratio due to pretreatment by a fixed film reactor, chemical pretreatment, or influenced by low BOD.sub.5 /TKN and low BOD.sub.5 /TP industrial wastes.